The invention relates to a method of welding hollow cylindrical bodies, in particular container bodies, in which the bodies pass through a guide tool comprising a plurality of rollers before reaching a welding station. The invention also relates to an apparatus for performing the method.
In a known process, hollow cylindrical bodies, eg. container bodies and in particular can bodies, are formed from flat sheet-metal blanks, either by butting the edges together or by forming an overlapping region which is then joined by roller seam welding. In the roller seam welding process, the bodies to be welded are, as is known, guided through a large number of rollers ahead of the welding zone, the rollers conforming to the body to be welded, which define a passage surrounding the body or a guide tool, eg. according to CH-A 671904. Such roller guide tools maintain the shape of the body and force the longitudinal edges of the body into engagement with a Z-rail which establishes the butting or overlapping of the longitudinal edges required for welding. Roller seam welding is then performed by feeding the blank through the guide tool, and with welding rollers acting on the overlap, usually with the use of wire electrodes. For the welding of container bodies, a series of such bodies are guided in succession through the guide tool to the welding rollers.
On initial start-up of the welding machine and on every start following a machine stop, the problem then arises that, at high rates of travel (over approximately 90 meters per minute), the first bodies run into each other, ie. the gap between the first and second (and possibly third) bodies cannot be set to the optimum. For these bodies, moreover, the slippage which occurs between the welding rollers and the bodies is relatively severe. This is revealed by the fact that the tin mark left by can bodies on the spent electrode wire is longer than the body length: for example, with a body length of 125 mm at a rate of travel of 138.8 m/min it is 139 mm for the first body, 133 mm for the second body, and a constant 128 mm for the subsequent bodies. Such slippage of the initial bodies is undesirable and is detrimental to weld quality.
Therefore the object which lies at the basis of the invention is to provide a welding method in which these drawbacks do not occur, or occur only to a limited degree.